Use CSS3 flex to lay out Files.app components.

Use CSS3 flex to lay out Files.app components.

This CL includes:
- Simple replacement by following rules.
    s/display: -webkit-box/display: flex/
    s/-webkit-box-orient: vertical/flex-direction: column/
    s/-webkit-box-orient: horizontal/flex-direction: row/
    s/-webkit-box-flex: 1/flex: auto/
    s/-webkit-box-flex: 0/flex: none/
    s/-webkit-box-pack: start/justify-content: flex-start/
    s/-webkit-box-pack: end/justify-content: flex-end/
    s/-webkit-box-pack: center/justify-content: center/
    s/-webkit-box-align: center/align-items: center/
    s/-webkit-box-align: baseline/align-items: baseline/
    s/-webkit-box-align: stretch/align-items: stretch/
- Flex items in "-webkit-box" don't shrink by default, but ones in "flex" shrink by default. So some flex items are specified "flex: none;" additionally.
- "vertical-align: middle;" doesn't work for flex items, so specify "align-items: center;" for their parents.
- Some position adjustment taking care of difference between "-webkit-box" and "flex". (commented inline)

BUG=387568

Review URL: https://codereview.chromium.org/376783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281985 0039d316-1c4b-4281-b951-d872f2087c98